The present invention in general relates to a high frequency power amplifier using a heterojunction bipolar transistor (hereinbelow, called an HBT) made of InGaP/GaAs or the like and a communication apparatus using the high frequency power amplifier. More particularly, this invention relates to a high frequency power amplifier used in the case of performing an output power control, typified by a CDMA (Code Division Multiple Access) system or the like.
Recently, a high frequency amplifier for use in a portable telephone or the like is required to have higher efficiency to satisfy both small size and light weight of a battery and a longer speech communication time. Particularly, in a high frequency amplifier for a digital radio communication system, in order to reduce power consumption, an RF output power is controlled as necessary. Consequently, high efficiency and high linearity are requested over a wide range of an RF input power.
FIG. 1 is a circuit diagram showing a conventional high frequency power amplifier using an emitter follower transistor type constant voltage source bias circuit. As shown in FIG. 1, in a high frequency amplifier using a bipolar transistor, an emitter follower transistor 12 is used as a voltage source for supplying a base current to an amplifying transistor 11. In the circuit, in order to cancel out a temperature change in a voltage Vbe between the base and emitter of the amplifying transistor 11 and in a voltage Vbe between the base and emitter of the emitter follower transistor 12, transistors 13 and 14 at two stages are provided.
Each of the voltage Vbe between the base and emitter of the amplifying transistor 11 and the voltage Vbe between the base and emitter of the emitter follower transistor 12 is 1.3V. In the high frequency power amplifier having such a configuration, therefore, a reference voltage higher than 2.6V has to be supplied from the outside to a bias circuit.
Meanwhile, recently, a portable telephone system operated on a power supply voltage of about 3V by using a lithium ion battery, NiCd (nickel cadmium) battery, or the like is being realized. In such a system, the voltage of a control IC for outputting the reference voltage is becoming lower. It is, therefore, difficult to make the reference voltage higher than 2.6V.
Consequently, it is considered to apply a bias by using a resistor without using the emitter follower transistor. FIG. 2 is a circuit diagram showing the configuration of the high frequency power amplifier in the case where a resistor is used for a base bias circuit. In the high frequency power amplifier, a resistor 15 is connected to the base of the amplifying transistor 11. In the circuit, in order to cancel a temperature change in the voltage Vbe between the base and emitter of the amplifying transistor 11, a transistor 16 for temperature compensation is provided.
In the bias circuit shown in FIG. 2, however, a base bias current which flows via the resistor 15 changes in proportion to an RF output power of the amplifier. A base bias voltage therefore largely fluctuates and an operating point is not constant. Consequently, a problem such that an adjacent leakage power ratio deteriorates occurs.
In a state where the RF output power is low, the base bias current is reduced as compared with that at the time of a maximum RF output, so that a voltage drop amount of the resistor 15 decreases. It makes the base bias voltage rise, and the operation of the amplifier becomes Class A operation. A problem such that the efficiency deteriorates arises.
Further, the reference voltage output of the control IC is generally a CMOS output. Consequently, there is also a problem such that it is difficult for the control IC to supply the base bias current of 10 to 20 mA required by an amplifier of about 1W output.
It is an object of this invention to provide a high frequency power amplifier which can operate with high efficiency over a wide range of an RF output by changing a collector-bias voltage of an amplifying transistor and a communication apparatus using the high frequency power amplifier.
The frequency power amplifier according to one aspect of this invention comprises a detecting unit which detects a collector output power (or base input power) of an amplifying transistor, a voltage changing unit which changes a collector voltage of the amplifying transistor in proportion to the detected power. Furthermore, a resistor for a base bias of the amplifying transistor is connected to the voltage changing unit.
According to the above-mentioned aspect of this invention, the collector voltage of the amplifying transistor is changed in proportion to the RF output power and the DC power consumed by the amplifying transistor is consequently controlled, so that high efficiency can be obtained over a wide range of the RF output. Since the base bias control of the amplifying transistor can be performed interlockingly with the control of a collector voltage source of a large current capacity, a base bias control terminal becomes unnecessary.